Human serum collected in previous studies and in this project will be studied to evaluate the relationships between immunity, on the one hand, and infection and colonization, on the other, of pregnant women and newborn infants by group B streptococci. Antibody will be measured by radioimmunoassay and opsonic activity will be measured by a phagocytic assay. The objectives are better understanding of naturally occurring immunity to the group B streptococci and better methods of predicting susceptibility and risk of streptococcal infection.